This invention relates to apparatus and method for disposition of radioactive waste materials.
While the invention may be used for the packaging and disposition of various types of radioactive or chemically dangerour wastes, it can be exceptionally advantageously employed in the disposition of radioactive wastes as they occur in nuclear electric power generating stations.
In known boiling water reactor plants, water is passed through the nuclear reactor through suitable conduits and is heated and converted to steam by the heat of the reactor. This steam passes through a turbine that drives an electric generator, then is recondensed and returned to the reactor to be reheated and converted into steam and so on. This is a closed system.
In pressurized water reactor systems, there is a first closed conduit loop extending through the nuclear reactor and then outside the reactor where it passes through a heat exchanger. The body of water or other liquid in the first closed loop is heated by the nuclear reactor, but the liquid is kept at such a high pressure, usually several thousand pounds per square inch, that it is not converted into steam or vapor. There is a second closed conduit loop carrying a body of water that extends through the heat exchanger out of contact with the liquid in the first loop. The water in this second circuit and thereby converted into steam which passes in the usual manner through a turbine driving an electric generator after which the steam is condensed and returned to the heat exchanger where it is again reheated.
In each such system water of the greatest possible purity is used. Nevertheless, minor amounts of impurities are present in he water initially introduced into the conduits. Further impurities appear because of the action of the water on the metal of pipes and conduits through which it passes. These impurities may become radioactive, particularly in water that passes through the reactor. Traces of cobalt leached out of stainless steel piping are particularly troublesome, since cobalt develops an intense form of radioactivity having a long half life.
In both boiling water and pressurized water reactors, it is a practice to subject the water to cleaning action by passing it through beds of ion-exchange resins. Such resins are of known composition. In general, they act similarly to natural or synthetic materials used in commercial water treating equipment. Through chemical and filtering action, they remove dissolved and suspended impurities, thus maintaining the water at the desired high purity. Otherwise the accumulation of impurities could result in scaling on the heat transfer surfaces, which would result in loss of efficiency or difficulty in operations. Resin particles of one type widely used for this purpose are those approximately 20 mesh in size. Resin particles of another widely used type are much smaller, approximating 300 mesh in size.
The contaminated resins of either type are removed from the water treating vessel by sluicing them out with water. The resulting slurry or dispersion is collected in a waste resin tank at the plant. The apparatus disclosed hereafter is particularly well adapted for disposition of this type of radioactive slurry.
Another type of radioactive waste material that may be handled by the present invention is "evaporator bottoms". These include concentrated liquid wastes from the plant, such as solutions containing boric acid, borax, sodium sulphate and the like which are used in the control of the reactor or for washdown of equipment for decontamination. Evaporator bottoms are also obtained from the collected water that is used for washing down portions of equipment or plant, wash water for employees, and chemical laboratory liquid wastes. This water containing radioactive impurities is temporarily stored and periodically portions of it are evaporated, leaving a solution or dispersion containing reactive materials in water that are known as "evaporator bottoms".
Stringent laws, rules and regulations govern the disposition of radioactive wastes and their transportation over highways, on railroads and by other modes of transportation. In general, the material must be shielded so that radiations emanating from the material do not exceed maximum levels established by the laws and regulations. Furthermore, it is desired that in a case of an accident causing dumping of a radioactive load, there should be no fluidic materials that can penetrate the grounds or mix with streams or ground water and cause radioactive contamination. It has therefore been proposed to provide a mixture of resin particles containing radioactive material, cement as a solidifying agent, and water in a container such as a steel drum, and to allow the mixture to solidify in the drum.
However, prior systems for putting radioactive materials into a drum or other container in general require that operators and maintenance personnel be exposed to radiation, even though such system may be intended to protect personnel. For example, the operators in many cases must go into areas containing radiations to open drums or close them or to insert nozzles in the drums or to handle the drums in storage. In some systems an operator may stand behind a shield wall, but must extend his arms into a radioactive zone, and expose his head to see, to connect pipes for feeding radioactive material. If spills occur, the operator must go into the radioactive zone to clean up spills. In prior operations where drums are stacked in multiple layers in decay storage areas, the operator must often go into such areas to place planks between the layers. Maintenance men must go into radioactive areas to work on equipment requiring maintenance at intervals, such as conveying equipment, motors, and switches. The total amount of radiation to which personnel can be safely exposed is limited by physiological reasons; therefore, personnel must be controlled as to their duties, and the amount of radiation to which they are exposed frequently checked to avoid their exposure to an excessive amount of radiation that can adversely affect health. Moreover, in operation of the nuclear plant, if an emergency should arise correction of which would require exposure of operators or maintenance men to radiation during a time when all available men had reached their limits of radiation tolerance, a shutdown of the plant might be necessary or other adverse consequences might result because of lack of operators or maintenance men having safe radiation tolerances.
Moreover, prior systems do not in general provide desired close control to insure that proper amounts of radioactive material, cement, or water are put into the drum to insure proper solidification of drum contents. It is imperative to avoid improper loading of the drum or mixing of drum contents.
Moreover, prior systems can on occasion spill radioactie materials on the outside of the drums or on the floor. If the spill is on the drum, it is necessary to decontaminate the drum prior to shipment. If the spill is on the floor, then a certain amount of dust can be generated as the material dries. Such dust, which is radioactive, could find its way through the plant and thus make the plant unsafe because of radioactivity. Spilled materials also can collect in floor drains and clog them. Prior systems for putting radioactive material into drums in general have loaded drums in an open space, so there was no way of containing or taking care of the problems caused by spills of radioactive material.
Some previous systems have numerous operating mechanical parts requiring periodic maintenance, such as motors and electrical switches, in radioactive areas. Maintenance of such equipment can expose personnel to considerable radiation.
Previous equipment loads drums containing radioactive material onto trucks or casks in a haphazard fashion, and thus, not loading the truck or cask to full capacity, would lose lading and could cause damage to the drums or drum enclosure.
Previous systems, because of loss of electrical power or air pressure or improper handling of the drum handling means, could topple a drum or cause irregularities in operation which could cause spillage of radioactive material. Previous systems did not provide for an accurate weight of resin to a weight of cement ratio in order to insure that the drum contents would be properly solidified with the most economical use of cement and with the lowest transportation costs. In prior systems that filter the resin in the drums to remove water, costs are understandably higher for the drum because of the added equipment contained therein.
Some prior systems mix radioactive resins, cement and water in a mixture outside of a drum. This involves exposure of considerable amounts of equipment to radioactivity and possibilities of considerable exposure of personnel to radioactivity. Moreover, the mixer must be cleaned after each use, which is difficult because the cement sticks to the mixer; moreover, the mixer will become radioactive and hence unsafe in time considerably shorter than the life of the plant, necessitating replacement expense. Some prior systems mix these materials in the drum; but if an open top drum is used, considerable spillage occurs during mixing, and if the drum is rolled about its lengthwise axis to mix its contents a core of poorly mixed materials is formed in the center of the drum.
Most if not all prior systems lack fail-safe features to prevent unsafe conditions in the event of failure of operations of any portion of the equipment.